"Your alliance will not achieve anything. Amass all your weapons and face us. We welcome you," he said in a 28-minute speech, one of three videos posted by the Islamists on YouTube.

Troops from Nigeria have been backed by soldiers from Chad, Cameroon and Niger in recent weeks because of increased concerns about Boko Haram's threat to regional security.

In the second of the latest videos, images of the leader of the Islamic State group, Abu Bakr al-Baghdadi, are shown along with archive footage and a voiceover recalling a battle between British colonial soldiers and fighters from the Sokoto Caliphate in northern Nigeria.

Shekau has namechecked al-Baghdadi before but appears to be positioning Boko Haram in a wider jihadi context by showing the Sokoto Caliphate, which was dismantled by the British in the early 20th century.

"We never rose up to fight Africa. We rose up to fight the world," he said.

"We are going to fight the world on the principle that whoever doesn't obey Allah and the Prophet to either obey or die or become a slave."

On Sunday, Boko Haram militants waged twin attacks in the town of Diffa in southeast Niger, opening a new front in its offensive after repeated attacks in Cameroon's far northern region.

The six-year uprising has become a regional crisis and on Saturday, Nigeria, Chad, Niger, Cameroon and Benin agreed to muster 8,700 troops, police and civilians to fight the group.

But Shekau dismissed the size of the force, which had previously been set at about 7,500.

"You send 7,000 troops? Why don't you send 70 million? This is small. Only 7,000? By Allah, it is small. We can seize them one-by-one. We can seize them one-by-one," he said in Arabic.